Impact

A remote, unauthenticated attacker may be able to execute arbitrary code with root privileges.

Solution

Apply an update
D-Link has released firmware updates to address the vulnerabilities in affected routers. Please see their announcement.
If you are unable to update your device, please see the following workarounds:

Restrict Access

As a general good security practice, only allow connections from trusted hosts and networks. Additionally, you may wish to disable remote administration of the router.